Pupil diameter is one of the most frequently assessed objective indices of the pharmacodynamic effects of opioids in humans. The pupillary effects of opioids are easily measured photographically in adults and are a highly sensitive measure of the presence and extent of physical dependence and withdrawal. Measures of pupillary response are often collected along with self-reported and observer-rated effects to provide a comprehensive profile of opioid effects. Infants exposed to opioids in utero are born physically dependent and often suffer from withdrawal (i.e., neonatal abstinence syndrome~ NAS) that is severe enough to require pharmacological treatment. Observer- rated scales are the only tools available to evaluate the need for and effectiveness of pharmacological treatment for NAS in both clinical and research settings and shortcomings of these instruments (e.g., rudimentary validation, long training and administration times) have led to calls for more sensitive, objective measures of NAS. We have developed a protocol to measure pupil diameter in opioid-exposed neonates. Pilot data (N=10) using this protocol indicate neonatal pupils respond to opioids in the same manner as adults and suggest that pupil diameter may be a more sensitive measure of NAS compared to observer-rated scales. Thus, the next step is to fully test the reliability and validity of pupil diameter in opioid-exposed neonates. Two studies are proposed to meet this overarching aim. The first study will formally test the reliability of ou pupil photography protocol in 40 opioid-exposed neonates. The second study will test the validity of pupillary changes in neonates as an index of opioid effects using the extensive adult literature on the pupillary effects of opioids as a guide. One-hundred ten opioid-exposed neonates and 30 non-exposed neonates will provide data for four analyses of validity. First, pupil diameter measurements collected over the first 24 hours after delivery will be examined to see if they discriminate between opioid-exposed and non-exposed neonates, testing discriminant validity. Second, among the opioid-exposed neonates, correlations between changes in pupil diameter and changes in observer-rated NAS scale scores over the first 24 hours after delivery will be examined, testing concurrent validity. Third, opioid tolerance and the time course of pupillary effects after opioid administration will be examined among the opioid-exposed neonates who require pharmacological treatment (estimated n= 35), testing construct validity. The fourth and final analysis will examine predictive validity by testing whether changes in pupil diameter in the first 24 hours after delivery predicts which neonates went on to receive pharmacological treatment. In sum, establishing the reliability and validit of pupil diameter in opioid-exposed neonates has potential to substantially improve assessment and treatment of opioid-exposed neonates in both clinical and research settings, reducing morbidity, shortening hospital stays, and decreasing costs.